The Syrian Democratic Forces' attack from north of the city comes as Syrian government troops close in from the south.

US-backed fighters in Syria have announced the launch of a new offensive against ISIL in the oil-rich Deir Az Zor province, bringing them into a race with government forces marching in the same direction to oust the armed group from its last major stronghold in the country.

The Syrian Democratic Forces (SDF), a coalition of Kurdish and Arab fighters, are attacking from the north of Deir Az Zor city, aiming to clear ISIL from territory east of the Euphrates River.

Ahmad Abu Khawlah, head of SDF's Deir Az Zor Military Council, made the comments at a press conference on Saturday in the village of Abu Fas in Syria's northeast Hasakeh province.

He said his forces had begun fighting to push ISIL out of remaining territory in Hasakeh province and further south in Deir Az Zor in eastern Syria.

The announcement came as Syrian government forces, backed by Russian air power and Hezbollah fighters, closed in from the south of Deir Az Zor city in a separate military campaign against ISIL.

The dueling battles for Deir Az Zor highlight the importance of the key eastern province, which has become the latest epicentre of the international war against ISIL, raising concerns of an eventual clash between the two sides.

Battle for Raqqa

The Syrian conflict, which started as a popular uprising against President Bashar al-Assad in 2011, has drawn the military involvement of several world powers. Peace talks have repeatedly failed to bring an end to the war.

The SDF's new push comes as its fighters are still involved in a major offensive to capture Raqqa from ISIL, also known as Islamic State of Iraq and the Levant, or ISIS.

Three months into the battle, they have taken around 60 percent of the city, and much more difficult urban fighting still lies ahead.

Earlier this week, Syrian troops and their allies reached Deir Az Zor, breaking a nearly three-year-old ISIL siege on government-held parts of the city in a major breakthrough in their offensive against the group.

In a victory statement, the Syrian military said Deir Az Zor will be used as a launching pad to recapture the remaining ISIL-held areas along the border with Iraq.

The troops' arrival to Deir Az Zor city brings Syrian forces and their allies a step closer to controlling the oil-rich eastern province and its capital bordering Iraq. The region has some of Syria's largest oil fields, whose revenue is vital to the state's dried coffers.

Syria's military command announced on Saturday they had ousted ISIL fighters from the province's Taym oilfield, southwest of Deir Az Zor city, on the other side of the Euphrates.

They also seized part of a main highway running from Deir Az Zor downstream to the city of al-Mayadeen, to which many ISIL fighters have retreated, said the Syrian Observatory for Human Rights, a UK-based monitor said.

That advance would help block potential ISIL reinforcements from al-Mayadeen, it added.

'Wars fought within wars'

SDF officials say the timing of the new push is not related to government forces reaching Deir Az Zor earlier this week, and was planned months in advance.

"Deir Az Zor is a main connection point and a very important geographic area," said Syrian Kurdish official Nawaf Khalil, who is in Germany but frequently visits northern Syria. He said the battle for Raqqa requires fewer fighters now than it did in its earlier stages.

The US-led coalition fighting ISIL said in an email to The Associated Press that the SDF "will decide when the conditions are right for an offensive".

Asked about concerns of a possible clash between the SDF and Syrian troops, the coalition said: "We urge all forces to concentrate their efforts on our common enemy (IS)."

So far, the rival campaigns have mostly stayed out of each other's way, and Washington has welcomed Syrian troops' fight against ISIL.

Both the US and Russia have an interest in avoiding a clash between the SDF and Syrian forces and may devise a strategy that will allow both sides to share control of the vast province.

US officials have suggested they are not seeking a confrontation with Assad's forces.